Thermal–Electrochemical Coupled Catalysis for Ammonia Synthesis under Atmospheric Pressure on Protonic Ceramic Electrolysis Cells Cooperated with Iron Catalysts

The electrochemical synthesis of ammonia from N2 and H2/H2O using solid oxide electrolysis cells (SOECs)/protonic ceramic electrolysis cells (PCECs) has received widespread attention and is recognized as a promising alternative to the traditional Haber–Bosch process without thermodynamic limitation. Unfortunately, the ammonia synthesis performance of SOECs/PCECs is significantly hindered by the sluggish kinetics of nitrogen reduction reaction. In this work, we propose a simple strategy to improve the electrochemical ammonia formation rate by coupling the PCEC cathode with iron-based thermocatalysts. Under PCEC operation conditions, the overall NH3 formation rate consists of the thermal NH3 formation rate, λthermal, measured under open circuit voltage (OCV) and the electrochemical NH3 formation rate, λelectrochem, obtained from the difference of values measured under OCV and electrolysis. The results show that with the introduction of iron-based catalysts, the thermal–electrochemical coupled catalysis achieved a NH3 formation rate of 16.6 μmolNH3 h–1 cm–2 at 700 °C under 1.2 V, one order of magnitude higher than that under conventional electrochemical catalysis. This work demonstrates that the ammonia synthesis during thermal–electrochemical coupled catalysis is mainly attributed to the N2 activation and reaction between H2 and N2 over the thermocatalyst in addition to the direct electrochemical synthesis of NH3. The results also show that under PCEC operation conditions, thermally formed NH3, i.e., λthermal, contributes significantly to the overall electrochemical synthesis of ammonia from N2 and H2/H2O on iron-based catalysts.